Optical imaging assembly and system with optical distortion correction

ABSTRACT

An optical imaging assembly is provided, having an optical axis; an object axis defined by an object being imaged; an aperture stop disposed on the optical axis; a light-transmissive sleeve enclosing the object axis, being disposed in object space defined by the object axis; and at least three refractive lens elements being arranged between the object and the aperture stop without any other intervening optical component, at least one of the elements having surfaces having at least one of cylindrical and acylindrical prescription, with an image plane, wherein the object being imaged lies within the sleeve.

CROSS-REFERENCE

The present application is a Continuation application of U.S. patentapplication Ser. No. 15/704,931 filed Sep. 14, 2017, which is aContinuation application of U.S. patent application Ser. No. 15/133,486filed Apr. 20, 2016, now U.S. Pat. No. 9,791,676, which is aContinuation application of U.S. patent application Ser. No. 14/169,633filed Jan. 31, 2014, now U.S. Pat. No. 9,360,658, all of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to optical imaging andmeasuring systems, and more specifically to such a system used forcalibrating fluid flow to a medical infusion pump.

BACKGROUND

One way to measure the rate of flow of a fluid is to cause the fluidflow to be in a continuous stream of drops of known volume, and thencount the number of droplets per unit time to deduce the flow rate. Thisapproach is very coarse because it has a measurement granularity equalto the volume of the droplets, and it assumes that the volume of eachdroplet is the same as it detaches from its orifice. Indeed, this “dropcounting” approach has measurement accuracy that is inadequate for manyapplications, such as medical infusion. The granularity problem can beeliminated if the volume of the droplets can be measured in real-time asthe droplets form and detach from the supporting orifice.

One way to measure the volume is to capture a two-dimensional image of apendant drop suspended from its orifice, and then measure its widthalong several points from the tip of the droplet to the orifice. Ifrotational symmetry is assumed, the droplet can be represented as aseries of stacked disks where the volume of each disk is V=πH(Width/2)²,where H is the distance between points along the axis of rotation. Thevolume of the drop is the sum of the volume of all the disks. To obtaingood droplet volume accuracy, it is important to obtain good estimatesof the width of the droplet. The rate of fluid flow can then be moreaccurately determined by measuring the time rate of change of dropletvolume, by for example, collecting and processing a series of images inquick succession, such as a series of video images.

Complicating the imaging process is the fact that the pendant drop of aninfusion tube is enclosed in a generally cylindrical drip chamber thatintroduces enormous amounts of optical distortion in the direction thatthe width of the droplet is to be measured. Further complicating mattersis that splashes and condensation can cause fluid droplets to form onthe inner surface of the drip chamber that can occlude or partiallyocclude the edge of the droplet from the image. Lastly, due tomanufacturing, assembly, and even usage processes, the imaging assemblymust be able to tolerate changes in distance between the axis of thependant droplet and the lens without causing an appreciable change inthe calculated volume of the droplet.

SUMMARY

Accordingly, an optical imaging assembly is prescribed that is opticallyfast, corrects for optical distortion introduced by a sleeve co-axialwith an axis of the object, and is telecentric in object space. Thepresent assembly employs combinations of cylindrical or acylindrical,and spherical or aspherical lens elements to correct optical distortionand other aberrations. In addition, the present disclosure relates to anoptical imaging assembly for use with an infusion tube, or, moreparticularly, for imaging the pendant drop within an infusion tube. Thepresent optical imaging assembly corrects for the optical distortioncaused by the infusion tube, is optically fast so that droplets andother artifacts residing on the wall of the infusion tube are out offocus and not imaged by the imaging system, and is telecentric so themagnification of the object is substantially independent of the distancebetween the object and the first lens element.

According to aspects illustrated herein, there is provided an opticalimaging assembly, including: an optical axis connecting an object planeand an image plane; an object axis within the object plane andperpendicular to the optical axis; a first optical element with asubstantially planar input surface and acylindrical output surface wherethe axis of acylindricity intersects the optical axis and is parallel tothe object axis; a second optical element with a substantially planarinput surface and acylindrical output surface where the axis ofacylindricity intersects the optical axis and is parallel to the objectaxis and the acylindrical output surface of the second optical elementis spaced away from the acylindrical output surface of the first opticalelement; a third optical element with input and output surfaces havingrotational symmetry and centered on the optical axis; an aperture stop;and a fourth optical element with input and output surfaces havingrotational symmetry and centered on the optical axis.

More specifically, an optical imaging assembly is provided, including anoptical axis, with an object axis, having a light-transmissive sleeveenclosing the object axis, telecentric in object space, having at leastthree refractive lens elements, in two of the lens elements, at leastone of the elements having surfaces with at least one of cylindrical andacylindrical prescription, with an image plane, wherein the object beingimaged lies within the sleeve.

In one embodiment, an assembly includes four lens elements arranged in amanner such that the resulting optical imaging assembly is able tocorrect for large amounts of optical distortion, is telecentric inobject space, has an f-number of 1.5 or less. Two of the lens elementshave aspherical prescriptions, and the other two lens elements haveacylindrical surfaces, wherein the two acylindrical surfaces areseparated from one another. The optical imaging assembly is well adaptedfor use in a liquid flowmeter system in which the fluid flows in aseries of droplets enclosed in a drip chamber.

In another embodiment, an imaging assembly is configured for removingoptical distortion from an image generated by an object located within alight transmissive sleeve. The assembly includes a first optical elementacting in conjunction with a second optical element; both opticalelements have cylindrical and/or acylindrical surfaces that togetherremove optical distortion from the image.

In yet another embodiment, an optical imaging assembly is provided,having an optical axis; an object axis defined by an object beingimaged; an aperture stop disposed on the optical axis; alight-transmissive sleeve enclosing the object axis, being disposed inobject space defined by the object axis; and at least three refractivelens elements being arranged between the object and the aperture stopwithout any other intervening optical component, at least one of theelements having surfaces having at least one of cylindrical andacylindrical prescription, with an image plane, wherein the object beingimaged lies within the sleeve.

In yet another embodiment, an imaging assembly is provided, having anoptical axis; an object axis defined by an object being imaged; anaperture stop disposed on the optical axis; four lens elements disposedon the optical axis, at least three of the four lens elements beingarranged between the object and the aperture stop without any otherintervening optical component; and a light-transmissive sleeve beingdisposed in object space defined by the object axis. The imagingassembly has an optical speed f-number of 1.5 or less, two of the fourlens elements have aspherical prescriptions, and the other two of thefour lens elements have acylindrical surfaces, and the two acylindricalsurfaces are separated from one another.

In still a further embodiment, an optical imaging assembly is providedhaving a light-transmissive sleeve arranged on an optical axis andconfigured to enclose an object, four refractive elements arranged inseries on the optical axis and each having an input surface and anoutput surface, and an aperture stop disposed on the optical axis,wherein the aperture stop is arranged between the third and fourthrefractive elements. At least one of the input surface of the thirdrefractive element, the output surface of the third refractive element,the input surface of the fourth refractive element, and the outputsurface of the fourth refractive element have radial symmetry.

The optical imaging assembly may further include an image plane arrangeon the optical axis, and the fourth refractive element may be arrangedon the optical axis between the third refractive element and the imageplane. The fourth refractive element may be two lens elements.

The at least one of the input surface of the third refractive element,the output surface of the third refractive element, the input surface ofthe fourth refractive element, and the output surface of the fourthrefractive element having radial symmetry may be a spherical surface.

The at least one of the input surface of the third refractive element,the output surface of the third refractive element, the input surface ofthe fourth refractive element, and the output surface of the fourthrefractive element having radial symmetry is an aspherical surface.

Both the input surface of the third refractive element and the outputsurface of the third refractive element may have radial symmetry.

Both the input surface of the fourth refractive element and the outputsurface of the fourth refractive element may have radial symmetry.

Both the input surface of the third refractive element and the outputsurface of the third refractive element may have radial symmetry. Theradial symmetry may be a spherical surface or an aspherical surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature and mode of operation of the present optical imaging assemblywill now be more fully described in the following detailed descriptiontaken with the accompanying drawing figures, in which:

FIG. 1 is a schematic top-view of the present optical imaging assembly;

FIG. 2 is a schematic side-view of the present optical imaging assembly;

FIG. 3 is an isometric view of the object, the sleeve about the object,and objective lens elements of the present optical imaging assembly;

FIG. 4 is a top-view ray-trace plot showing how a fan of raysoriginating at the edge of the field in the object plane propagatethrough the optical imaging assembly to the image plane;

FIG. 5 is a representative image of a pendant drop within a sleevehaving inner surface droplets in which the optical imaging assembly isnot optically fast;

FIG. 6 is a representative image of a pendant drop within a sleevehaving inner surface droplets in which the optical imaging assembly isoptically fast;

FIGS. 7A, 7B, and 7C, are a prescription of an embodiment of the presentoptical imaging assembly, created by the Zemax lens design program;

FIGS. 8A and 8B are graphs from the Zemax lens design programillustrating the amount of optical distortion of the optical imagingassembly in the directions parallel to the object axis and perpendicularto the object axis, respectively, with a cylindrical sleeve locatedabout the object;

FIG. 9 are spot diagrams from the Zemax lens design program showing thesize and shape of the images produced by the present optical imagingassembly in which the object consists of delta-functions at six fieldlocations with a sleeve located about the object;

and

FIG. 10 is a block diagram illustrating how the present optical imagingassembly is used in a flow-rate measurement system.

DETAILED DESCRIPTION

At the outset, it should be appreciated that like drawing numbers ondifferent views identify identical, or functionally similar, elements ofthe present disclosure.

Furthermore, it is understood that the present disclosure is not limitedto the particular methodology, materials, and modifications asdescribed, and any of these may, of course, vary. It is also understoodthat the terminology used herein is for the purpose of describingparticular aspects only, and is not intended to limit the scope of thepresent disclosure, which is limited only by the appended claims.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which the present disclosure belongs. Although any methods,devices, or materials similar or equivalent to those described hereincan be used in the practice or testing of the present disclosure,example methods, devices, and, materials are now described.

FIG. 1 is a schematic top-view of optical imaging assembly 100, whichincludes an optical axis 102, a first lens element 112 having an inputsurface 134 and an output surface 136, a second lens element 114 havingan input surface 138 and an output surface 140, a third lens element 116having an input surface 142 and an output surface 144, an aperture stop118, and a fourth lens element 120 having an input surface 146 and anoutput surface 148. The object plane 104 is perpendicular to the opticalaxis 102 and contains at least a portion of the object being imaged suchas the pendant drop 152 shown in FIG. 3. Object space 101 also includesa sleeve 110 having an axis of rotation 108, the axis of rotation 108also being substantially coincident with a rotationally symmetric objectsuch as the pendant drop 152 shown in FIG. 3. The sleeve 110 ispreferably substantially cylindrical, is contemplated as being slightlycone-shaped with a slope of approximately 0.5 to 5.0° for facilitatingthe molding process, and has an inner surface 130 and an outer surface132. The image produced by the optical imaging assembly 100 lies inimage plane 106.

Also shown in FIG. 1 is a key to the axes in which the Z-axis is takento be the optical axis 102, the Y-axis is perpendicular to the Z-axis inthe plane of the drawing, and the X-axis is perpendicular to the Z-axisand perpendicular to the plane of the drawing. The object plane 104 isin the X-Y plane at Z=0.

Each of the components listed above will be described more fully withreference to FIGS. 1, 2, and 3. The first lens element 112 is arefractive optical element having a substantially planar input surface134 and a cylindrical or acylindrical output surface 136. Planarsurfaces are less costly to produce than non-planar surfaces, and shouldbe used whenever possible to reduce the manufacturing costs of theoptical imaging assembly 100. Furthermore, making input surface 134planar facilitates placement and replacement of the sleeve 110 in frontof the optical imaging assembly 100 so that different objects can beinstalled in front of the optical imaging assembly 100 as needed. Outputsurface 136, being cylindrical or acylindrical, has optical power in theY-axis direction and little or no optical power in the X-axis.

The second lens element 114 is a refractive optical element having asubstantially planar input surface 138 and a cylindrical or acylindricaloutput surface 140. Planar surfaces are less costly to produce thannon-planar surfaces, and should be used whenever possible to reduce themanufacturing costs of the optical imaging assembly 100. The outputsurface 140, being cylindrical or acylindrical, has optical power in theY-axis direction and little or no optical power in the X-axis.

In FIGS. 1, 2, and 3, the cylindrical/acylindrical surfaces are shown toreside on the output surfaces, 136 and 140, although they could resideon the input surfaces, 134 and 138, or a combination of input and outputsurfaces such as input surfaces 134 and output surface 140 or outputsurface 136 and input surface 138.

In FIGS. 1, 2, and 3, both cylindrical/acylindrical surfaces haveoptical power in the Y-direction (i.e., perpendicular to the opticalaxis 102 and perpendicular to the object axis 108), although the opticalpower could instead be in the X-direction (i.e., the direction parallelto the object axis 108), or one cylindrical/acylindrical surface canhave optical power in the Y-direction and the othercylindrical/acylindrical surface can have optical power in theX-direction.

The third lens element 116 is a refractive optical element having aspherical or aspherical input surface 142 whose center of rotation issubstantially coincident with the optical axis 102. Similarly, theoutput surface 144 is spherical or aspherical and also has a center ofrotation substantially coincident with the optical axis 102.

An aperture stop 118 is placed between the third lens element 116 andthe fourth lens element 120. The aperture stop 118 can be fabricatedfrom opaque thin sheet material, such as metal or plastic sheeting. Theaperture of the aperture stop 118 is nominally round, but can have othershapes as well such as square, rectangular, hexagonal, octagonal, or anyshape made from arbitrary lines segments and arcs. The aperture of theaperture stop 118 is nominally centered on the optical axis 102. Adistance from one side to an opposing side of the aperture of theaperture stop 118 can be between 1 mm and 100 mm when measured throughthe optical axis 102.

All refractive lens elements 112, 114, 116, and 118 are contemplated asbeing made from glass or polymer such as acrylic, polycarbonate, orpolystyrene, although in general materials having a higher refractiveindex such as polycarbonate or polystyrene provide for greater opticalpower, which in turn facilitates a more compact design in which thedistance from the object plane 104 to the image plane 106 is reduced. Ifthe choice of material is polymer, any or all of the lens elements 112,114, 116, and 118 can be made from an injection molding process,compression molding process, injection-compression molding process, oreven diamond turned. If the choice of material is glass, any or all ofthe lens elements 112, 114, 116, and 118 can be fabricated with atraditional glass grinding and polishing process, an advanced polishingprocess such as MRF (magneto-rheological finishing), a diamond turnedprocess, or with a molding process.

The thicknesses of each of the refractive lens elements 112, 114, 116,and 118, as measured from the apex of the input surface to the apex ofthe output surface along the optical axis, can be from between 1.0 and25.0 mm. The perimeter of the refractive elements 112, 114, 116, and 118can be rectangular, such as shown, for example, for first lens element112 in FIG. 3, or circular such as shown, for example, for third lenselement 116 in FIG. 3, or they can have any number of arbitrary curvesand sides to facilitate manufacturing. A distance from one side to anopposing side of any or all refractive lens elements 112, 114, 116, and118, can be between 10 mm and 200 mm when measured through the opticalaxis 102.

If any or all of the refractive lens elements 112, 114, 116, and 118 aremade with a molding process, then mounting, alignment, or attachmentfeatures can be incorporated into the lens element during thefabrication process.

Due to Fresnel reflection, each surface of the refractive lens elements112, 114, 116, and 118 will back-reflect approximately 4% of the lightincident upon it, resulting in diminished light throughput and straylight that can form glints or other artifacts in the image that cancorrupt the image processing process. An antireflective coating can beinstalled onto some or all of the surfaces of the refractive lenselements 112, 114, 116, and 118 to reduce the Fresnel surfacereflectance to less 1%. The antireflective coating can be a broad-bandantireflective coating, or it can be a multi-layer interference filmstack.

Furthermore, the coating on the input surface 134 of the first opticalelement 112 should have abrasion resistance properties because the dripchamber 300 will need to be replaced at the start of every infusion.Also, abrasion resistance is beneficial since the drip chamber is inclose proximity to the input surface 134, which can be scratched ordamaged when the drip chamber 300 is installed.

Surrounding the object plane 104 and the object 152 is the sleeve 110.In the preferred embodiment, the substantially cylindrical sleeve 110 isnot part of the optical imaging assembly 100, but instead resides in theobject space 101 and is used to enclose, encapsulate, or otherwisecontain the object 152. The sleeve 110 is substantially transparent ortranslucent to the light being used to image the object 152, and can bemade from a polymer such as acrylic, polycarbonate, polystyrene, orvinyl. The sleeve 110 can be part of an infusion administration set,such as that made by Baxter International, Inc. If the sleeve 110 ispart of an infusion administration set, then the sleeve is known as adrip chamber, and the object 152 is a pendant drop residing within thedrip chamber and centered or nearly centered on the optical axis 102.The sleeve drip chamber 110 is nominally centered on the object axis108, and has an inner radius of 7.8 mm and an outer radius of 8.8 mm,although the sleeve drip chamber can have other radii in the range of1.0 mm to 100 mm.

The sleeve drip chamber 110 introduces severe optical distortion alongthe Y-axis that must be compensated by the optical imaging assembly 100for accurate measurement of the width of the object 152. That is, forbest results, the image of the object 152 at the image plane 106 shouldbe substantially free from optical distortion.

The sleeve drip chamber 110 is typically fabricated with a low-costinjection molding process. To reduce fabrication costs, the mold usedcan have surface imperfections that impart surface imperfections intothe cylindrical sleeve that can appear in the image of the object 152.Furthermore, it is expected that the sleeve drip chamber 110 can haveseam lines, flow lines, and particulate imperfections that can allappear in the image.

When fluids are flowing through the sleeve 110 in operation, i.e., whenthe object 152 droplets are forming and detaching inside the sleeve dripchamber, splashes from the fluid reservoir at the bottom of the sleevedrip chamber can settle on the inner surface 130 of the sleeve withinthe field of view of the optical imaging assembly 100. Furthermore, overlong periods of time, the fluid flowing through the sleeve 110 canevaporate and subsequently condense on the inner surface 130 of thesleeve 110 within the field of view of the optical imaging assembly 100.This condensation can appear as a collection of closely-spaced droplets,and significantly impair the ability of a conventional imaging assemblyto image the interior of the sleeve 110. Both the aforementionedsplashes and condensation are shown in FIG. 3 as sidewall droplets 154.

Another challenge facing the optical imaging assembly 100 is theplacement of the sleeve 110, or more particularly the location of theobject axis 108 and object 152 relative to the optical imaging assembly100. That is, due to instabilities and the flexibility of a vinyl sleevedrip chamber 110, the distance between the object axis 108 and the inputsurface 134 of the first lens element 112 can vary by severalmillimeters. This dimensional problem is exacerbated whenever one sleevedrip chamber 110 is replaced with another like component as typicallyoccurs when one infusion ends and another begins. Since themagnification of a lens typically varies with varying object distance,the varying magnification will cause the image size to vary and thecalculated volume of the pendant drop object 152 to be inaccurate, whichwill in turn cause the computed flow rate to be inaccurate as well.

The preceding paragraphs have illustrated the need for the opticalimaging assembly 100 to have the following set of characteristics: 1)the optical imaging assembly 100 must be telecentric in object space sothe magnification does not change with varying object-to-input surfacedistance; 2) the optical imaging assembly 100 must be optically fast, onthe order of F/1.5 or faster, so that sidewall droplets 154 and otherundesirable artifacts within the sleeve drip chamber 110 are out offocus and do not appear in the image; and 3) the optical distortionintroduced by the sleeve 110 is removed by the optical imaging assembly100. An additional desirable characteristic is that the optical imagingassembly 100 be as compact as possible, meaning, for example, that thedistance between the object plane 104 and the image plane 106 is small,such as less than 150 mm. The present optical imaging assembly 100 hasthese four desirable features, whose functions are described in thefollowing paragraphs.

Telecentricity in object space 101 is that condition where the ray thatleaves the object 152 propagating parallel to the optical axis 102passes through the center of the aperture stop 118. In FIG. 4, thatparticular ray, also called the chief ray, is seen to be ray 164C, whichleaves the object at location 160 in a direction substantially parallelto the optical axis 102, and subsequently passes through the aperturestop 118 at location 119. Note that the location 119 is substantially atthe center of the aperture stop 118, and the chief ray 164C intersectsthe optical axis 102 at the location 119.

The object space telecentricity condition is determined by the opticalpower of the third lens element 116, and the optical distance betweenthe third lens element 116 and the object plane 104, as well as theoptical distance between the third lens element 116 and the aperturestop 118.

As described earlier, the drip chamber 110 introduces crippling amountsof optical distortion that are removed by the optical imaging assembly100. This optical distortion compensation is achieved with the firstoptical element 112 acting in conjunction with the second opticalelement 114. Both of these optical elements have cylindrical and/oracylindrical surfaces (i.e., output lens surface 136 and output lenssurface 140) that together remove the optical distortion from the image.Initial attempts at designing the distortion-compensation lens assemblyutilized only one optical element having one or two cylindrical and/oracylindrical surfaces; intuitively this approach seemed reasonable sincethe sleeve 110 is only one optical component (external to the lensproper), and the distortion it introduces should be counteracted withonly one lens element having a cylindrical or acylindrical surface.However, it was found that all designs that utilized only one elementhaving a cylindrical or acylindrical surface could not be made opticallyfast and/or telecentric, or suffered from poor image quality.

In addition to requiring two lens elements for optical distortioncorrection (namely the first lens element 112 and the second lenselement 114), the cylindrical/acylindrical surfaces of these two lenselements are preferably physically separated from one another by aconsiderable distance, such as 4 mm or more. This separation allows forthe distortion-correction characteristics of onecylindrical/acylindrical surface to be leveraged against the secondcylindrical/acylindrical surface. That is, because the twoacylindrical/cylindrical surfaces (e.g., 136 and 140) are separated,their aberration-compensating effects are not simply additive, butinstead interact producing higher-order distortion-compensation terms.This interaction is one of the key components of the present assembly100.

The optical imaging assembly 100 is preferably optically fast, as notedearlier, so obscurations residing within the sleeve 110 drip chamber, orobscurations residing on either the inner surface 130 or outer surface132, are out of focus and do not appear in the image. These obscurationsdo not appear in the image if the optical imaging assembly has anoptical speed less than approximately F/2.0, or preferably less thanF/1.5.

It is typically not difficult to design a lens having an f-number of 2.0or less, although the design of such a lens does become difficult if theobject or image field size is large, or if substantial aberrations arepresent and must be eliminated. Both of these conditions are present inthe present operational environment, and the optical imaging assembly100 preferably provides good image quality over the entire field at therequisite optical speed. This is accomplished with the third opticalelement 116 and the fourth optical element 120, both of which have inputand output surfaces that have radially symmetric optical power. Thesefour surfaces can be spherical in nature, although better image qualitycan be obtained if they are aspherical, such as an asphere described byan eighth-order polynomial, although lower order polynomials—such assixth order—can be used as well.

The diameter of the aperture of the aperture stop 118 also plays a rolein defining the optical speed of the optical imaging assembly 100.Generally speaking, the greater the width of the aperture the faster thelens, although a larger aperture generally allows more highly aberratedrays to reach the image resulting in poorer image quality.

To summarize, the first lens element 112 and the second lens element 114are used to correct the optical distortion introduced by the sleeve 110;the third lens element 116 and the aperture stop 118 are used to controlthe object-space telecentricity of the optical imaging assembly 100, andthe third lens element 116 and the fourth lens element 120 with theaperture stop 118 are used to provide good image quality with lowf-number.

FIG. 3 shows one application of the optical imaging assembly 100 inwhich the fluid flow rate of an infusion administration set is measured.In such a setup, the object is the pendant drop 152 suspended from anorifice 150, both of which are substantially located on the object axis108. During operation the pendant drop 152 grows in size as the infusedfluid flows, then detaches from the orifice 150 when it reaches itsterminal weight, and then grows and detaches repeatedly until thedesired volume of fluid has been administered. Since the volume of thedroplet is less than a milliliter, several thousand drops grow anddetach over the course of an infusion.

During the course of an infusion, droplets 154 can form on the innersurface of the sleeve drip chamber 110. These droplets 154 can resultfrom splashes from the falling droplet landing in the fluid reservoir atthe bottom of the drip chamber. Since the course of an infusion can lastseveral hours, fluid can evaporate from the pendant droplet 152 and fromthe reservoir of fluid at the bottom of the drip chamber. If thetemperature of the inner surface 130 is low enough, then some of theevaporated fluid can condense on the inner surface 130 and presentthemselves as droplets 154.

If the optical speed of the optical imaging assembly 100 is relativelylow (i.e., high f-number), then the droplets 154 will be in focus, orpartially in focus, at the image plane 106. For example, FIG. 5 shows animage of the pendant droplet 152 in the presence of inner surface 130droplets 154 when the speed of the optical imaging assembly 100 is onlyf/5.6. Note that the images of the droplets 154 are easily discernible.Worse, some of the droplets 154 lie at the edge of the image of thependant drop 152, which, to the image processing software, will make thesize of the pendant drop 152 appear to be greater than it actually is,and will cause the fluid flow measurement calculations to produceinaccurate results.

FIG. 6 shows is an image of the pendant drop 152 with the same set ofdroplets 154 residing on the inner surface 130 as was made for the imageof FIG. 5. However, the image of FIG. 6 was made with an optical imagingassembly 100 having an optical speed of f/1.4. Note that images ofdroplets 154 are barely noticeable and the edge of the image of thependant drop 152 has good contrast and fidelity. The image processingsoftware will be able to compute the size of the pendant drop 152 withgood accuracy.

One such embodiment of the optical imaging assembly 100 was designedwith Zemax (Radiant Zemax, LLC, Redmond Wash., USA). The prescription ofthe assembly is given in FIGS. 7A, 7B, and 7C. Highlights of the designshown in FIG. 7A include: a total track of 108.1 mm (the distance fromthe object plane 104 to the image plane 106), a stop radius of 7.5 mm, aworking F/# of 1.40, a maximum object field width of 8.8 mm, amagnification of −0.526, and the wavelength of the light is 825 nm. Theimage quality was set to be optimized at six object field locations,being, in X,Y pairs in millimeters: (0.0, 0.0), (4.0, 0.0), (0.0, 3.0),(0.0, 5.5), (8.8, 0.0), and (6.0, 3.5).

In FIG. 7B it is seen that the optical model consists of an object “OBJ”plane and image “IMA” plane, an aperture stop “STO”, and eleven othersurfaces. Surface 1 is a dummy surface used by Zemax for telecentricityoptimization. Surfaces 2 and 3 are the inner surface 130 and outersurface 132 of the transparent sleeve 110, which is made from PVC.Surfaces 4 and 5 are the input surface 134 and the output surface 136 ofthe first lens element 112, which is made from polystyrene (POLYSTYR).Surfaces 6 and 7 are the input surface 138 and the output surface 140 ofthe second lens element 114, which is also made from polystyrene.Surfaces 8 and 9 are the input surface 142 and the output surface 144 ofthe third lens element 116, which is also made from polystyrene. Lastly,surfaces 11 and 12 are the input surface 146 and the output surface 148of the fourth lens element 120, which is made from polystyrene as well.

Further down in FIG. 7B, and in FIG. 7C, it is seen that the inputsurface 134 of the first lens element and the input surface 138 of thesecond lens element both have no curvature and are in fact planar.Output surface 136 of the first lens element and the output surface 140of the second lens element both have acylindrical prescriptions. Bothsurfaces of the third lens element 116 and the fourth lens element 120are aspherical.

FIGS. 8A and 8B are plots of optical distortion present in the image inthe X direction (parallel to the object axis 108) and the Y direction(perpendicular to the object axis 108). In the X direction, thedistortion is only a few tens of microns out to a field distance ofabout 5 mm. In the Y direction, the distortion is only a few tens ofmicrons out to a radial field distance of about 4 mm. Note that in the Ydirection, the distortion is undefined at radial field distances greaterthan the radius of the inner surface 130 of the sleeve 110.

FIG. 9 is a collection of image spot diagrams for the six object fieldlocations noted earlier, and optimized by Zemax. Note the scale is 400um, which is the height and width of each of the six graphs. The RMSwidth of each of the six spots is substantially less than 100 um. Sincea pixel of a CCD or CMOS image sensor is typically 10 um or less, theedges of the pendant drop 152 object will be imaged across about tenpixels, which is ideal for localizing the edge of the image of theobject with sub-pixel accuracy with advanced image-processingalgorithms.

FIG. 10 shows how the present optical imaging assembly 100 can be usedas part of a flowmeter 200 of a medical infusion device to measure therate of flow of the infused fluid. The flowmeter includes a bag 312 orcontainer of fluid that is to be infused, a pendant drop 152 of infusionfluid whose rate of flow is to be measured, a drip chamber 300 with exitport 310 and an exit tube 308 carrying infusion fluid to a patient.

As seen in FIG. 10, the flowmeter 200 also includes a backlight 202 thatis used to illuminate the pendant drop 152 of infusion fluid, theoptical imaging assembly 100, an image sensor 204 located at the imageplane 106, a communication bus 212 at the output of the image sensor 204carries image data to a digital processing device 206, which in turn isconnected through a communication bus 220 to a memory element 208 whichis used to store image data 216, other data 214, and processinginstructions 210.

In operation, infusion fluid slowly leaves the fluid bag 312 and forms apendant drop 152 within the drip chamber 300. Next, the backlight 202 isused to illuminate the pendant drop 152 through the sleeve 110 of thedrip chamber 300. The light 203 that passes through the sleeve 110 isthen collected by the optical imaging assembly 100 which then forms animage of the pendant drop 152 on the image sensor 204. The output of theimage sensor 204 is pixelated image data in the form of atwo-dimensional array of integer data, where the integer datacorresponds to the brightness of the image at each location of thearray. This digital array of brightness data is then transmitted overthe communication bus 212 to the processor 206 that processes the imagearray data to 1) find the edge of the image of the pendant drop 152within the array, and 2) compute the volume of the pendant drop 152 atthe particular instant the image was captured by the image sensor 204.Knowing the precise time at which successive images are captured by theimage sensor 204, and accurately computing the volume of the pendantdrop 152 in each successive frame allows the time rate of change of thependant drop 152 to be calculated, which is the rate of flow of thefluid.

It was mentioned earlier that a compact embodiment of the opticalimaging assembly 100 is more desirable than an embodiment that is notcompact. In some configurations, a more compact embodiment can beachieved by inserting a fold mirror into the assembly, such as betweenthe third lens element 116 and the fourth lens element 120. Typicallythe fold mirror will be centered on the optical axis 102, and tilted ata 45° angle with respect to the optical axis 102 so the imaging path isbent 90°. This can reduce the width of the envelope that the opticalimaging assembly 100 occupies by about 30%, although it will increasethe size in an orthogonal direction. But this increase in size in anorthogonal direction generally will not increase the overall size of theflowmeter 200, because other flowmeter components in the orthogonaldirection will constrain the size of the flowmeter 200 in thisdimension.

The magnification was mentioned earlier in connection with FIG. 7A to be−0.526. The minus sign means that the image is inverted with respect tothe object. Indeed, the apex of the pendant drop 152 in FIG. 3 is seento be in the downward direction, while the image of the pendant drop inFIGS. 5 and 6 are seen to be in the upward direction. The magnitude ofthe magnification, 0.526 means that the size of the image is only 52.6%the size of the object, which is desirable because a smaller and lessexpensive image sensor 204 can be used as part of the flowmeter 200. Thesign of the magnification of the optical imaging assembly 100 willgenerally be negative, although the magnitude of the magnification canbe tailored to the size of the image sensor 204 and can be between 0.1and 10.0.

The wavelength of light was mentioned earlier in connection with FIG. 7Ato be 825 nm. The wavelength of the light used must be producible by thebacklight 202, transmissible by all of the optical elements of theoptical imaging assembly 100, transmissible by the sleeve 110, and theimage sensor 204 must be responsive to it. The image sensor 204 isgenerally a silicon device, and is responsive to wavelengths between 400nm and 1100 nm; the backlight can consist of one or more LED (lightemitting diode) sources, which can emit light between 400 nm and 900 nm;and most refractive optical elements can transmit light in the visibleand near IR spectral bands, including the wavelengths from 400 nm to1100 nm. Therefore, the range of light wavelengths that can be used withthe optical imaging assembly 100 can be from 400 nm to 900 nm.

As seen in FIG. 4, the center thickness of the fourth lens element 120is rather thick, being 8.32 mm thick as prescribed in FIG. 7B. Polymerlens elements having a large thickness can be difficult to mold withgood fidelity due to the large amount of shrinkage that the centralportion of the lens element undergoes relative to the thinner outerportion as the lens cools after being molded. To remedy this, the fourthlens element 120 can be divided into two separate thinner lens elements.This has the disadvantage of increased material and assembly costs, butalso provides two additional degrees of freedom that can be used toimprove the image quality with the addition of the two surfaces of afifth lens element.

Having thus described the basic concept of the invention, it will berather apparent to those skilled in the art that the foregoing detaileddisclosure is intended to be presented by way of example only, and isnot limiting. Various alterations, improvements, and modifications willoccur and are intended to those skilled in the art, though not expresslystated herein. These alterations, improvements, and modifications areintended to be suggested hereby, and are within the spirit and scope ofthe invention. Additionally, the recited order of processing elements orsequences, or the use of numbers, letters, or other designationstherefore, is not intended to limit the claimed processes to any orderexcept as may be specified in the claims. Accordingly, the invention islimited only by the following claims and equivalents thereto.

What is claimed is:
 1. An optical imaging assembly, comprising: alight-transmissive sleeve arranged on an optical axis and configured toenclose an object; four refractive elements arranged in series on theoptical axis and each having an input surface and an output surface; andan aperture stop disposed on the optical axis, wherein the aperture stopis arranged between the third and fourth refractive elements; and,wherein at least one of the input surface of the third refractiveelement, the output surface of the third refractive element, the inputsurface of the fourth refractive element, and the output surface of thefourth refractive element have radial symmetry
 2. The optical imagingassembly as set forth in claim 1 further comprising an image planearrange on the optical axis, and wherein the fourth refractive elementis arranged on the optical axis between the third refractive element andthe image plane.
 3. The optical imaging assembly as set forth in claim 2wherein the fourth refractive element comprises two lens elements. 4.The optical imaging assembly as set forth in claim 1 wherein the atleast one of the input surface of the third refractive element, theoutput surface of the third refractive element, the input surface of thefourth refractive element, and the output surface of the fourthrefractive element having radial symmetry is a spherical surface.
 5. Theoptical imaging assembly as set forth in claim 1 wherein the at leastone of the input surface of the third refractive element, the outputsurface of the third refractive element, the input surface of the fourthrefractive element, and the output surface of the fourth refractiveelement having radial symmetry is an aspherical surface.
 6. The opticalimaging assembly as set forth in claim 1 wherein both the input surfaceof the third refractive element and the output surface of the thirdrefractive element have radial symmetry.
 7. The optical imaging assemblyas set forth in claim 1 wherein both the input surface of the fourthrefractive element and the output surface of the fourth refractiveelement have radial symmetry.
 8. The optical imaging assembly as setforth in claim 7 wherein both the input surface of the third refractiveelement and the output surface of the third refractive element haveradial symmetry.
 9. The optical imaging assembly as set forth in claim 8wherein the radial symmetry is a spherical surface.
 10. The opticalimaging assembly as set forth in claim 8 wherein the radial symmetry isan aspherical surface.